Advanced Studies of Membrane Fouling

: Investigation of Cake Fouling Using Fluid Dynamic Gauging

Abstract

Membrane filtrations are widely used in process industries but are almost always limited by fouling, a highly studied and significant problem. This is defined as unwanted material deposited on a membrane surface or within its pores, which can significantly impair performance and/or reduce operating life. The strategies to mitigate fouling include cleaning in place, modified membranes, and optimisation of operating conditions. In order to correctly select or target improvements to any such measures a detailed mechanistic understanding of the fouling process is important, which requires more than just performance data from unit operations. One key mechanism is that of cake fouling, which describes the build-up of particle layers on the surface of the membrane. Its growth and physical properties are difficult to assess.In this project the technique of fluid dynamic gauging (FDG) has been explored as a means to study cake fouling. This simple, yet robust method allows for estimation of thickness and strength of cake fouling at high concentrations and opacity, without any prerequisite knowledge of feed properties. Studies described herein focused on microfiltrations through cellulosic membranes. FDG was used to track cake growth during filtrations of polydisperse yeast suspensions (which contained large agglomerates), demonstrating its capability to work with non-ideal, food-like substances. Later studies used more predictable suspensions of hollow glass spheres, which were used to assess various filtration models. The most effective was found to be an interpretation of the critical flux laws, which were used to successfully identify pore fouling during filtrations of Kraft lignin, an observation supported by FDG measurements.Another novel achievement of this project was the development of an automated apparatus for performing FDG in cross-flow membrane filtration. This allowed for much faster acquisition of results, and demonstrated the potential for its development into an autonomous system capable of making thickness measurements on the fly during filtrations. The most reliable protocol for determining cake growth rates was by repeated filtrations in which destructive thickness testing was performed at selected time points. This was because continuous or even repeated thickness measurements during a single filtration were found to cause too much disturbance to the fouling layer.Computational fluid dynamics was used to simulate shear stress profiles on the fouling layer, while also providing a more accurate means to calibrate the automated apparatus. Erosion caused by FDG readings, when viewed under a microscope, was found to conform to the shear stress profiles predicted by simulations.